선입금 means "deposit" or "pre-payment" (선=early, pre-, 입=deposit, entry, 금=amount, fund). 일반 is literally "general", so most likely "general sales" here. So it could be that the first period is when people can place deposits (to secure a deal or something), and the second is for general sales. But that's just my wild guess. You should go by the rules of the site and your experience with them (especially since deposits are an often used ploy to rip people off).